<%= form.fields_for field.name, page_part, :child_index => child_index do |page_part_builder| %>
  <% field.children.each do |field| %>
    <%= field.render :form, parent_controller, page_part_builder.object, :builder => page_part_builder %>
  <% end %>
  <%= page_part_builder.hidden_field :id, :data => { :acts => 'id' } if page_part_builder.object.persisted? %>
<% end %>
